I'm pretty sure the Rope of Stone is a magic item. I do konw that it's a rope that turns to stone on command, increasing durability and weight limit and all. The thing is, stone is rather fragile, especially stone as long and thin as a rope. I don't see it holding up to the stress of trying to harpoon somebody with it; it'd be very easy to just break the stone rope and run off with your harpoon. You're better off leaving it in rope form, which is harder to break.